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Head-to-head evidence comparison — which supplement is right for you?
Glycine vs Vitamin D3: Vitamin D3 has the stronger overall evidence (7.5 vs 6.5/10); they're alternatives for perceived sleep quality — the best pick depends on your goals. Take the 60-second quiz for a pick tailored to your goals.
Vitamin D3 wins 2 of 3 categories. Both are solid choices — the best pick depends on your specific goals.
Verdict
Too few graded studies
1 of 2 studies with measurable effects showed benefit.
Top outcomes
Verdict
Probably helps
7 of 14 studies with measurable effects showed benefit.
Top outcomes
Shared outcomes (1)
Outcomes where both Glycine and Vitamin D3 have evidence — compare verdict strength side-by-side.
3g before bed (no cited trial states the dose it used)
30-60 minutes before bed
Powder (dissolves easily, tastes sweet)
2000-4000 IU daily
Morning with breakfast
D3 (cholecalciferol) softgel or liquid
and 1 more not shown
First few nights
Next morning
Immediate
4-8 weeks
4-8 weeks
2-4 weeks
4-8 weeks
The effect of glycine administration on the characteristics of physiological systems in human adults: A systematic review
GeroScience (2024) · Systematic review
The nervous system demonstrated the most positive effects, including improved psychiatric symptoms from longer-term glycine administration in psychiatric populations.
Glutamatergic drugs for schizophrenia
The Cochrane database of systematic reviews (2006) · Meta analysis · n=358
NMDA receptor co-agonists glycine and D-serine showed some effects in reducing the negative symptoms of schizophrenia (n=132, SMD -0.66, CI -1.0 to -0.3, p=0.0004), but the magnitude of the effect was moderate.
Ingestion of a Whey Plus Collagen Protein Blend Increases Myofibrillar and Muscle Connective Protein Synthesis Rates
Medicine and science in sports and exercise (2025) · Rct · n=28
In addition, myofibrillar protein synthesis rates were higher in BLEND compared with PLA in both the rested (0.038 ± 0.008 and 0.031 ± 0.006%·h -1 , respectively; P < 0.05) and exercised (0.052 ± 0.011 and 0.039 ± 0.009%·h -1 , respectively; P < 0.01) leg.
Vitamin D supplementation vs. placebo and incident type 2 diabetes in an ancillary study of the randomized Vitamin D and Omega-3 Trial
Nature communications (2025) · Rct · n=22220
Ancillary analysis of VITAL: 22,220 adults without T2D randomized to vitamin D3 2000 IU/day or placebo, with a meta-analysis of 3 additional trials.
Vitamin D Supplementation for Patients with Dysmenorrhoea: A Meta-Analysis with Trial Sequential Analysis of Randomised Controlled Trials
Nutrients (2024) · Meta analysis · n=687
TSA revealed that the current RCTs provide sufficient information.
Vitamin D and respiratory tract infections
BMJ (2017) · Meta analysis · n=11321
25 RCTs, 11 321 participants aged 0 to 95 years. Vitamin D reduced acute respiratory tract infection overall (adjusted OR 0.88, 95% CI 0.81 to 0.96).
Both points come from a meta-analysis of double-blind cholecalciferol trials in people aged 60 and over (PMID 15886381). Trials at 700 to 800 IU a day cut hip fracture risk by 26% (RR 0.74) and non-vertebral fractures by 23%, plotted at 800 IU. Trials at 400 IU a day showed no benefit (hip fracture RR 1.15), and the WHI trial of 400 IU with 1,000 mg calcium did not significantly reduce hip fracture (PMID 16481635). Most of these trials also gave calcium, and the Cochrane review found vitamin D alone unlikely to prevent fractures while vitamin D with calcium gave a small reduction (PMID 24729336). Doses above 800 IU were not compared, so no optimal dose or ceiling is set.
AI-estimated from published studies. Interpret as directional guidance.
Vitamin D3 has a higher evidence score (7.5/10 vs 6.5/10) and wins in 2 of 3 categories.
For perceived sleep quality, Glycine has a higher relevance score (95 vs 60).
No documented interaction was found in the data searched. This does not establish that this exact combination is safe.
Ashwagandha, L-theanine, magnesium, saffron — what actually helps stress, and the safety caveats.
When to take what, with or without food, and which supplements compete vs pair well.
Melatonin, magnesium, glycine, L-theanine — tiered by evidence, plus a wind-down timeline.
The honest tier list — proven staples vs situational vs mostly marketing. The hub the other guides feed into.
The right pick depends on your goals. Answer a few quick questions for a personalised recommendation — or dig into the full evidence on each.